Field of the Invention
Integrated circuit devices known as Field-Programmable Gate Array (FPGA) devices are provided.
Description of the Related Technology
FPGAs are Integrated Circuit (IC) devices comprising a large number of logic blocks and input/output blocks, interconnected through a network of programmable interconnects, so that a high degree of flexibility is obtained in terms of the functionality of the device. Interconnect routing, through the programmable interconnects, is typically realized in the Front-End-Of-Line (FEOL) part of the FPGA's production process, which is the part wherein the FPGA's logic blocks are created on a semiconductor substrate. As each programmable interconnect circuit of a typical routing matrix typically requires a circuit of up to six pass transistors (see hereafter), each pass transistor being equipped with its own Static Random Access Memory (SRAM) cell, with each SRAM cell itself typically comprising 6 transistors, it is clear that the interconnect routing consumes a large area on the semiconductor surface.
FIG. 1A shows a typical routing matrix, each horizontal and vertical line representing a signal path within the FPGA, running between I/O blocks and/or logic blocks on the device. At a number of the crossings between a horizontal and vertical line, a programmable interconnect circuit 5 is present. A detail of each of the interconnect circuits 5 is shown in FIG. 1B. Each interconnect circuit comprises six pass transistors 6 whose state determines whether or not signals are “pass”-ed (=switched) from each of the connection points 1-2-3-4 to the other connection points. Each pass transistor is associated with a memory element 7 in the form of an SRAM cell (only one SRAM cell is shown), the SRAM cell being a memory element for storing the on-or-off state of the associated pass transistor. Each SRAM cell comprises 6 transistors. This yields 42 FEOL transistors per interconnect point or 2100 transistors for a 50 by 50 interconnect matrix of the type shown in FIG. 1. Some interconnect circuits do not enable signals to be “pass”-ed or routed in all six directions between the connection points, so that less than 6 pass transistors are needed. However, the majority of interconnect circuits in an FPGA does contain the 6 pass transistors, each provided with an SRAM cell. The SRAM cells and the pass transistors are produced in the FEOL, which represents a large surface area on the chip, i.e. an inefficient use of the semiconductor surface.